Chimneys

Medieval hall-houses were heated by means of a central fire in an open hearth. Smoke rose freely through the height of the hall to escape through the ridge, depositing soot on the roof timbers on its way. In high-status houses, open hearths began to be replaced from about the middle of the fifteenth century with contained fireplaces and chimneys. Their use was widespread by the sixteenth century.

The construction of the fireplace varied according to the building traditions of different regions. Stone was the commonest material, but in parts of west Wales especially, hoods and flues were sometimes made of wickerwork daubed with clay. In areas where timber framing was common, half-timbered fireplaces may also be found.

Chimneys rapidly became very visible status symbols. For richer house owners, this encouraged the development of ever more elaborate designs during the sixteenth and seventeenth centuries. The use of bricks in the lowland areas along the border with England, for example, led to the clustering of flues, each serving a separate fireplace, into star-shaped chimney stacks with moulded brickwork. In the west, tall single shafts were more common. In the nineteenth century, the significance of the chimney as a status marker in smaller cottages sometimes led to the construction of dummy chimneys where there was no fireplace.

During the eighteenth century the design of chimney stacks became simpler, and terracotta chimney pots began to be used widely. At their simplest, chimneys pots were formed either by hand or on a potter’s wheel. However, the Victorians developed many highly decorative designs using moulds, allowing very rapid mass production.

What to look for: 

 Cracked or damaged chimney pots

Suggested action:

Use binoculars to inspect pots for damage. Repair cracked pots and replace broken pots with exact copies. 

Chimney pots should never be permanently removed from buildings dating from the eighteenth century or later, where they were an original feature – even if the flue is disused.

Repair may, however, require the temporary removal of a pot. Cracks can be repaired using epoxy-resin adhesives, and the pot further strengthened by binding it with fine stainless steel wires. Re-setting weathered pots to turn the damaged side away from the prevailing weather may extend their life.

Discrete terminals or inserts may be added to help prevent rainwater entering the flue. Bird guards can also be fitted.

What to look for: 

Cracked or damaged flaunching 

Suggested action: 

Inspect flaunching with binoculars; if necessary, repair or renew with lime mortar.

Flaunching is the cement or lime cap that holds the chimney pot in position on top of the stack. The condition of flaunching is often difficult to assess from ground level, so try to get a view from another building or higher ground. If safe access is possible, carefully inspect the flaunching for cracks or other signs of deterioration.

Use lime mortar to repair or renew flaunching, giving it a sloping surface to shed water. If old flaunching needs to be removed, take care to avoid damaging the chimney pots.

What to look for: 

Vegetation growing on the stack 

Suggested action: 

Remove vegetation at the earliest possible opportunity.

Vegetation, particularly self-seeded shrubs, saplings or ivy, can cause severe damage if left to grow unchecked. Invasive growth that cannot be removed by hand without damaging the mortar joints should be treated with a systemic weed killer and left in place to die and fall away.

Growth of vegetation is often a sign that repointing is needed to eliminate cracks and holes where seeds can lodge.

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What to look for: 

Leaning or split chimney stack 

Suggested action:

Use binoculars to inspect the chimney for  cracks or other signs of deterioration. If  changes have occurred since the last  inspection, consider stabilizing or rebuilding  the stack.

The extreme stresses that affect chimneys both inside and out can cause them to split or to lean. Leaning usually occurs when the erosion of mortar on the weather side causes the brick or stonework to settle, while the crystallization of salts on the opposite, slower drying, side causes the mortar joints to expand.

Although stone chimneys on many historic buildings are not perfectly plumb, provided that they are structurally sound they should not be disturbed. It may also be possible to avoid disturbance of small brick chimney stacks suffering from minor cracks by binding them with wires to strengthen them. Rake out horizontal joints in the brickwork, insert stainless steel binding wires and repoint the joints with lime mortar.

It is likely that a badly cracked or unstable chimney will need to be rebuilt on a like-for-like basis. The chimney should be recorded before it is carefully dismantled so that the method of construction, number of courses and other details can be accurately replicated. As many of the original stones or bricks as possible should be re-used and bedded in a lime mortar.

The top few courses of the chimney are the most exposed and susceptible to decay. It is tempting to remove them, but chimneys should not be reduced in height as this can result in downdraughts, causing smoke to blow back into the room. It is particularly important to reinstate the corbelling – the line of projecting stone or brick – at the head of the stack as this serves a practical function. It helps to prevent rainwater,from running down the face of the stack, which would quickly wash out mortar joints, weakening the structure.

If you are in any doubt about the stability of a chimney consult a structural engineer.

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What to look for: 

Brick or stone stacks - cracked or eroded stones or bricks, damaged pointing 

Suggested action:

Use binoculars to inspect the stack for perished bricks or stones and defective pointing. If necessary, replace individual bricks and stones and repoint.

Bricks and soft stone can perish through rain, wind and frost. Cement pointing can worsen the problem by trapping moisture within the structure.

If only isolated bricks or stones have perished, they can be carefully cut out and replaced, ensuring that the replacements are a good match in strength and appearance.

Lime mortar should be used for repointing.

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What to look for:

Rendered stacks - cracked, hollow or missing render

Suggested action: 

Monitor the stack for the appearance of cracks or bulges in the render; repair or renew render with suitable materials.

Many stone chimneys were historically given an external protective coating of render. Renders were usually lime based, until early cements began to come into use in the early nineteenth century. The early cements were softer and more permeable than modern Portland cement, which can trap moisture in the stone or brickwork.

Small cracks in a lime render should be monitored, but they are unlikely to need immediate attention. An application of limewash can help to heal small cracks as it is derived from limestone, as is the lime in the render.

Larger cracks can indicate a problem with the underlying structure, and should be investigated. Once this has been ruled out, larger cracks for missing patches of render can be filled with lime mortar.

Cracks of any size in a cement-based render should be tackled at an early stage as they encourage water to be drawn into the underlying material. Since cement is highly impermeable, water that penetrates into the porous masonry of the chimney will be unable to evaporate. This can cause damp to appear inside the building lower down the chimney-breast. It can also lead to the decay of the masonry of the chimney stack itself and further failure of the render.

Damaged cement render should be completely removed, if practical, and replaced with lime render.

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What to look for: 

Limewashed stacks – flaking  or weathered limewash

Suggested action:

Renew limewash regularly.

In most parts of Wales, stone buildings, including their chimneys, were traditionally limewashed for practical, as well as aesthetic reasons. Limewash provides a protective coating, which acts as a barrier against penetrating damp. While providing a good weatherproof cover, it also allows any water within the structure to evaporate through the surface.

Limewash should be applied regularly, ideally every year, to be effective

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What to look for: 

Painted stacks – flaking, blistered or weathered paint

Suggested action: 

Repaint with a silicate microporous paint, or, if an impermeable paint layer is causing deterioration of the stack, investigate removal.

Microporous silicate paints will allow a building to breathe and may be used in situations where limewash may be difficult to apply. Surfaces should be repainted every 3 to 4 years